To fully appreciate the benefits of post-tensioning, it is helpful to know a little bit about concrete. Concrete is very strong in compression but weak in tension, i.e. it will crack when forces act to pull it apart. In conventional concrete construction, if a load such as the cars in a parking garage is applied to a slab or beam, the beam will tend to deflect or sag. This deflection will cause the bottom of the beam to elongate slightly. Even a slight elongation is usually enough to cause cracking. Steel reinforcing bars (“rebar”) are typically embedded in the concrete as tensile reinforcement to limit the crack widths. Rebar is what is called “passive” reinforcement however; it does not carry any force until the concrete has already deflected enough to crack. Post-tensioning tendons, on the other hand, are considered “active” reinforcing. Because it is prestressed, the steel is effective as reinforcement even though the concrete may not be cracked. Post-tensioned structures can be designed to have minimal deflection and cracking, even under full load.